I had a "white GSD" (turns out he was all husky) rescue from down south who they didn't tell me had a bite history. He ended up biting me badly enough to require and ER visit and stitches. When I called the "rescue" to tell them what happened, they notified me that they needed to take him back. I didn't know I had any options (but frankly, I didn't have the skill set at the time to deal with a dog with aggression issues), so I agreed. He was HW+ and I had paid a fortune for treatments, but he passed away a month or so after they took him back. He was a very troubled dog...I think of him frequently. The things I learned about that "rescue" after the fact were disturbing.
I fostered a lab with some major behavioral problems (ZERO boundaries and dominance issues). I had him for four weeks and he was an absolute nightmare. When my husband's grandmother became extremely ill, I had to pass him off to another foster in the organization (we had to go to CT to care for her and make final arrangements). I was VERY clear that he needed to go to an experienced foster. The dog would not have been manageable in any environment but one that was very strict and regimented and boot-camp-esque. They ended up giving him to an elderly couple looking for a lab. As anyone who knew the dog could have predicted, he was a holy terror and broke the lady's hip, bit the man, and was euth'd. I think of him frequently as well. This was with one of the more reputable rescues in NE, or so I thought.
Sometimes I feel like I failed Echo. I was too inexperienced to be able to recognize his training needs (he's fearful, soft, and very anxious) and I feel like I made so many mistakes in training that I exacerbated the problems.